Indy herds moisture all the way from the Indian Ocean, bringing drier or wetter winter-springs depending on whether he’s feeling positive or negative. He’s more active in the north.

Enso herds moisture from the Pacific Ocean towards Tasmania’s north and east. bringing less rainfall in an El Nino, and more rainfall during a La Nina.

Eastie scampers along the south-east coast of Australia, can go into action overnight, and his favourite seasons are autumn and winter. He can cause strong winds, heavy rains and lots of rough weather.

Southern Annular Mode

The SAM describes a north-south movement of the “roaring forties”, the belt of strong westerly winds that blow around the southern hemisphere between 40° and 50° south. It varies over periods of weeks or months.

Because these winds regularly blow over Tasmania and markedly influence rainfall patterns over the state, especially in the west during winter, changes in the SAM often lead to changes in rainfall patterns over Tasmania.

One measure of SAM is the Antarctic Oscillation Index, produced by the US National Weather Service. Typically, this index varies between -1 and +1, with more extreme values indicating a particularly abnormal event.

The SAM can be in a positive or negative phase.

Positive SAM phase

When the SAM is positive, the belt of strong westerly winds contracts towards the South Pole.

Indian Ocean Dipole

The Indian Ocean Dipole (IOD) is a measure of changes in sea surface temperature patterns in the northern Indian Ocean. These changes contribute to the formation of cloud bands.

The IOD is derived from the difference in sea temperature between the western Indian Ocean, near Africa, and the eastern Indian Ocean near northern Australia.

An IOD event usually starts around May or June, peaks between August and October, and then rapidly decays.

The Dipole Mode Index is a measure of the Indian Ocean Dipole. It is calculated from the difference in sea surface temperature anomalies in the western and eastern equatorial Indian Ocean. When the index is greater than 0.4 or less than -0.4, the IOD is active. It exceeds 1.0 (or is less than -1.0) only occasionally.

The impact of the IOD on weather and ways to predict its development are areas of active research.

Positive dipole

The IOD is positive when waters are warmer than normal near Africa and cooler than normal near Australia.

This leads to an increase in the easterly winds across the Indian Ocean and reduced cloudiness (and, therefore, rainfall) near Australia.

In Tasmania, the result is often below-average winter and spring rainfall in the northern half of the state.

In many years when the IOD is positive, the Pacific Ocean is in an El Niño state, further increasing the likelihood of below-average rainfall.

Negative dipole

The IOD is negative when waters are cooler than normal near Africa and warmer than normal near Australia.

So, a negative IOD usually leads to a wetter-than-usual winter and spring in most of Tasmania.

1983 and 1992 – positive and negative IOD years

The IOD was positive is 1983. Winter-to-spring rainfall in Tasmania was below average in most of the north, but near or even slightly above average in the south (Figure 6). This rainfall pattern is often, but not always, seen in positive IOD years.

In contrast, the IOD was negative in 1992. Winter-to-spring rainfall was above average across much of Tasmania (Figure 7).

Both 1983 and 1992 were neutral ENSO (El Niño - Southern Oscillation) years, so they more clearly show the effect of the IOD without the effect of ENSO.

La Niña

La Niña is associated with extensive cooling of sea surface temperatures in the central and eastern tropical Pacific.

We usually see a warming of the waters to the north of Australia and higher than usual winter and spring rainfall in the east and north of Tasmania.

It can also be wetter than usual in the north of Tasmania during summer.

El Niño Modoki

‘Modoki’ is Japanese for ‘similar, but different’ - as such, El Niño Modoki is a phenomenon under scrutiny. It is not yet clear whether El Niño Modoki is a different climate driver to conventional El Niño.

El Niño Modoki is associated with strong warming of sea surface temperatures in the central tropical Pacific and cooler sea surface temperatures in eastern and western tropical Pacific waters.

Frontal systems

Frontal systems, such as cold fronts, generally move from west to east across the Southern Ocean and vary in their intensity and speed.

As Tasmania is located around 42 °S in the “roaring forties”, cold fronts cross the state regularly.

These cold fronts are usually connected to a low pressure system over the Southern Ocean, which can pass very close to Tasmania or well to the south.

Fronts are more common in winter, and sometimes cross the state every day for more than a week.

A front almost always brings rain or showers to the west of the state, but the strength of the front determines how much rain there is and if the rain will reach the east coast.

More intense systems are generally associated with heavier rainfall; however, a slow-moving front can also bring heavy falls if the rain persists over an extended period.

Thunderstorms sometimes also accompany a frontal system.

Cold fronts generally bring a drop in temperature, which may be barely noticeable but can be very marked.

Summer-time cold fronts can cause temperatures to drop more than 15 °C in a day.

Winter-time fronts can bring snowfalls at sea level.

The hottest days usually occur when there are northerly winds ahead of a cold front.

The coldest nights occur when a high pressure system moves over the state, immediately behind a cold front.

Vigorous wind changes can accompany frontal changes. Winds may gust to over 100 km/h as a front crosses. Strong and damaging winds are almost certain when the associated low is very deep or close to Tasmania.

Very large swells can develop in the west and south if these lows are deep and move fast across the Southern Ocean.

August 2009 – a spate of cold fronts brings floods and damaging winds

A cold front crossed Tasmania almost every 24 hours between 19 and 31 August 2009.

Each front brought moderate-to-heavy rainfall to the west and north, which led to widespread and persistent river flooding.

Many of the fronts caused damaging winds to blow across the state. On the 31st, winds gusted to over 80 km/h across the state and over 100 km/h in several areas, reaching 174 km/h at Maatsuyker Island in the southwest.

Waves reached 15 metres in the west, there were thunderstorms in the southwest and snow fell to around 300 metres in the south.

High pressure systems and blocking highs

They prefer to sit over mainland Australia during winter but south of the continent during summer.

Their effect varies according to their position, intensity, movement and other weather systems nearby.

A high centred right over Tasmania will generally bring fine weather, light winds, and very cold nights (during winter). Fog will often form if there is enough moisture in the air.

Highs over the Great Australian Bight often cause showers or drizzle in the west and south, and cooler-than-normal temperatures.

As a high moves to the south of Tasmania, easterly winds develop over the state, often resulting in drizzle about the east, which can last all day.

When a summer-time high moves over the Tasman Sea, high temperatures and mostly fine conditions are the usual outcome, though there may be some light precipitation in the north and northeast.

Blocking highs are strong high-pressure systems that remain near stationary for an extended period of time.

They block the west-to-east progression of weather systems across southern Australia, and often form over the Tasman Sea.

In such a situation, north-to-northeast winds blow over Tasmania for several days and temperatures can climb steadily. Moisture will also increase, making fogs more likely and sea fog may form in the north and east. Days are mostly fine, though there may be some drizzle in the north and northeast or some isolated showers.

Cut-off lows

Cut-off lows are low pressure systems that break away from the main belt of low pressure that lies across the Southern Ocean.

They are associated with sustained rainfall and can produce strong, gusty winds and high seas.

If a cut-off low is slow-moving, rain may fall for extended periods and be heavy at times. Rain can fall over several days.

Cut-off lows can develop over Australia, to the east of Australia or over the Great Australia Bight.

East-coast lows are a type of cut-off low that forms over the Tasman Sea to the east of Australia. They can direct warm, moist air over Tasmania, sometimes bringing over 100 mm of rain to the east coast.

The strong and gusty southeast winds and large easterly swells often associated with east-coast lows can cause widespread damage.

December 2008

A cut-off low passed over Victoria early on 13 December 2008 (Figure 20), then intensified near Flinders Island, bringing heavy rain and strong and gusty southeast winds to the northeast of Tasmania.

Winds gusted over 100 km/h.

Rainfall of over 100 mm was common in the northeast, but little rain fell in the west (Figure 21).

Rivers in the northeast flooded and the rain and wind caused significant damage on Flinders Island.

Strong winds and low pressure combined with high tides to blow water over some coastal roads in the east.

Upper-level troughs and cloud bands

Though not as common as cold fronts or high pressure systems, upper-level troughs and cloud bands sometimes move over Tasmania and influence the weather.

The two phenomena are linked; a trough of low pressure is often (but not always) accompanied by cloud, and cloud bands can form when a trough of low pressure occurs in the upper atmosphere.

Cloud bands can also form when warm, moist tropical air originating over the Indian Ocean moves towards the pole (generally south-eastward), and is forced to rise over colder air in southern Australia. Such a cloud band is called a northwest cloud band, and these can affect Tasmania.

Upper-level troughs and cloud bands often bring rain to Tasmania. Rain may be statewide or confined to a particular area, depending on the circumstances.

When cloud covers Tasmania, sunlight is blocked during the day, leading to cooler than normal temperatures. But when the sun sets, heat is trapped near the surface so overnight temperatures are warm.